Industrially used resin materials include various types of resin such as polycarbonate, polyethylene and ABS resins. In addition, materials obtained from monomers through polymerization reaction, that is, synthetic rubbers such as ethylene-propylene-diene rubber (EPDM), nitrile-butadiene rubber (NBR), stylene-butadiene rubber (SBR) and the like can be regarded as resin materials in a broad sense. In this description, the term reading “resin material” is used as including synthetic rubbers. Then, a monomer material is polymerized in a polymerization apparatus to obtain a so-called solvent resin solution. The resulting solvent resin solution is made up of a nonvolatile polymer and a volatile substance. Removing the volatile substance from the solvent resin solution produces a resin material. The removal of such a volatile substance, that is, devolatilization can be executed by an extruder. As well known, an extruder is made up of a cylinder and a screw which rotates in the cylinder, and a hopper is provided on the cylinder for feeding a solvent resin solution into the cylinder. In a volatilization extruder, a plurality of vents are provided on the cylinder to devolatilize volatile substances. Specifically, a rear vent is provided rearwards of a hopper, and one or two or more front vents are provided forwards of the hopper. A single screw extruder having a single screw or double screw extruder having two screws may be used for such a volatilization extruder. When a solvent resin solution is introduced into the extruder from the hopper to rotate the screws, the solvent resin solution is kneaded and pushed forwards. When the solvent resin solution is kneaded and pushed forwards, volatile substances volatilize from the solvent resin solution and are then discharged from the plurality of vents. Namely, the solvent resin solution is devolatilized. The resulting resin material from which the volatile substances are removed is extruded from a die at a front end of the extruder. The resin material so extruded is then cut as required by a cutter to thereby obtain pellets which constitute a material used in an injection molding machine.
When the solvent resin solution is extruded by the extruder provided with the vents, part of the volatile substances which have volatized is discharged from the rear vent. As this occurs, an entrainment phenomenon is produced in which the resin material is caused to flow in a splash or particulate form in a reverse direction within a predetermined range by means of the pressure of the volatile substances which have volatilized. As this occurs, splash matters or particulate matters, which are so-called entrained matters, are scattered around the rear bent. Many of these entrained matters are sent forwards by the screw to be kneaded together with the resin material inside the cylinder, causing no problem. However, part of the entrained matters accumulates inside the rear vent. Once those entrained matters accumulate in the rear vent, there is caused a problem that the devolatilization capability of the volatilization extruder is reduced. Additionally, there is also caused a problem that the accumulated entrained matters fall into the cylinder to contaminate the resin material in the cylinder. To cope with these problems, it is necessary to prevent the accumulation of entrained matters in the rear vent. As one of countermeasures, there is a method in which a long distance is ensured between the hopper and the rear vent. In an extruder in which a long distance is ensured between a hopper and a rear vent, even though entrained matters are produced near the hopper, it becomes difficult for the entrained matters so produced to be scattered as far as the vicinity of the rear bent, thereby reducing the risk of those entrained matters accumulating in the rear vent. As a different countermeasure, the flow velocity of volatile substances which have volatilized is reduced by increasing the volumetric capacity of a portion of a cylinder between a hopper and a rear vent by increasing a bore diameter of the cylinder or increasing the flight channel of a screw at the portion of the cylinder. This is because a reduced flow velocity can reduce the momentum of entrained matters which attempt to flow in the reverse direction.
PTL 1 also proposes an extruder which prevents the accumulation of entrained matters in a rear vent. The extruder described in PTL 1 includes a cylinder which is made up of a plurality of cylinder blocks. A hopper, that is, a material feeding port is provided on a cylinder block which is provided with a rear vent. The material feeding port is opened to a lateral or lower side of a screw, so that a material solution, that is, a solvent resin solution is designed to be fed into the cylinder from the lateral or lower side of the screw. Volatile substances volatilize from the solvent resin solution inside the cylinder and flow upwards to be discharged to the outside from the rear vent which is upper side of the screw. When feeding the solvent resin solution into the cylinder, since the solvent resin solution is fed from the lateral or lower side of the screw into the cylinder, there is no such situation that the solvent resin solution being fed disrupts the discharge of the volatile substances which have volatilized. Namely, there is no such situation that the volatile substances which have volatilized prevent the devolatilization of the volatile substances. This can prevent the scattering of the resin material to thereby prevent the accumulation of entrained matters in the rear vent.